This invention relates to a hip prosthesis that is preferably implanted using bone cement, but can also be implanted without using cement.
Joint replacement surgery has become standard therapy in the field of orthopaedic surgery, yet the prostheses that have been used to date and the processes for anchoring them cannot be considered as giving the final solution to the problems that arise during surgery. For example, cemented joint replacement components very often destroy the cement socket, whereupon the prosthesis loosens. This has led to the construction of a number of prostheses that can be implanted without the use of cement. However, the known prostheses of this kind still have their drawbacks. It was, for instance, discovered that cement-free implants can cause pain shortly after surgery, followed by uncontrolled atrophy around the bone, which may, in turn, cause pathological fractures (see I. W. Brown and P. A. Ring, Osteolytic changes in the upper femoral shaft following porous-coated hip replacement, J. Bone Joint Surg. 67B, pages 218 to 221). Furthermore, cement-free prostheses have by no means reduced the number of loose implants, but rather increased it.
In experiments on the hip joint, especially the femur, it has been possible to show that the cement-free components cause considerable deformation of the bone when it is under strain. The greater the mass of the implant, or the greater the deformability of the individual bone, the higher the degree of deformation. Yet in cement-free implants it is imperative to ensure positive contact with the bone from the very beginning. This fact has led to the development of positive or anatomically adapted components (see J. Henssge, Metbode zur Entwicklung anatomisch richtiger Implantatkorper, in "Grenzschichtprobleme der Verankerung von Implantaten unter besonderer Berucksichtigung von Endoprothesen", edited by M. Jager, M. H. Hackenbroch, H. J. Refior, Georg Thieme Verlag Stuttgart, New York, 1981). One extreme aspect of this development is the so-called "custom-made prosthesis" as introduced by Mulier, for example (J. C. Mulier, Improvements of the Charnely technique of total hip replacement and future developments, Acta. Orthop. Belg. 52, pages 392 to 403). However, the design of this prosthesis made it particularly heavy. The bone reacted accordingly with especially severe deformation. However, bone deformation leads to surface movement and thus to bone resorption, which may cause the prosthesis to loosen.
Attempts have also been made to apply the findings on other implants to the field of prostheses. One of the results was that the principle of prestressing, which is one of the basic principles of osteosynthesis techniques, was introduced into the field of endoprostheses, as was the so-called press-fit principle (M. E. Muller, Total hip replacement: planning, technique and complications, in: Cruess, R. L. Mitchell, N. S.: Surgical management of degenerative arthritis of the lower limb, Chapter 10, pages 91 to 113, Lea & Febiger, Philadelphia 1975; M. E. Muller and B. Elmiger: Coxarthrose, 10-Jahres-Ergebnisse der sog. Setzholz-Totalprothese, Orthopade 8, pages 73 to 74, 1979; K. Zweymuller, Knochen- und Gelenkersatz mit biokeramischen Endoprothesen, Facultas, Vienna 1978). Yet, just like the rigid prosthesis components, the principle of prestressing could not induce the bone to react in any other way but with severe deformation.
Long-term studies in more or less stably cemented prostheses have indicated the superiority of cemented components. The fact that the bony bed had been slightly remodelled was a sign of uniform load transfer, and the positive bone contact proved that there was no relative movement in the interface. This implies that the implant caused only slight deformation of the bone (K. Draenert, Histomorphologische Befunde zur gedampften und ungedampften Krafteinleitung in das knocherne Lager, Vereinigung Nordwestdeutscher Orthopaden, 36th Annual Conference, Hannover, 15th to 18th Jun. 1986).